Method for increased workpiece throughput

ABSTRACT

A method is disclosed for speeding workpiece thoughput in low pressure, high temperature semiconductor processing reactor. The method includes loading a workpiece into a chamber at atmospheric pressure, bringing the chamber down to an intermediate pressure, and heating the wafer while under the intermediate pressure. The chamber is then pumped down to the operating pressure. The preferred embodiments involve single wafer plasma ashers, where a wafer is loaded onto lift pins at a position above a wafer chuck, the pressure is rapidly pumped down to about 40 Torr by rapidly opening and closing an isolation valve, and the wafer is simultaneously lowered to the heated chuck. Alternatively, the wafer can be pre-processed to remove an implanted photoresist crust at a first temperature and the chamber then backfilled to about 40 Torr for further heating to close to the chuck temperature. At 40 Torr, the heat transfer from the chuck to the wafer is relatively fast, but still slow enough to avoid thermal shock. In the interim, the pump line is further pumped down to operating pressure (about 1 Torr) behind the isolation valve. The chamber pressure is then again reduced by opening the isolation valve, and the wafer is processed.

REFERENCE TO RELATED APPLICATION

[0001] The present application is a divisional of U.S. application Ser.No. 09/749,648, filed Dec. 27, 2000, which claims the priority benefitunder 35 U.S.C. §119(e) to provisional application No. 60/194,227, filedApr. 3, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates generally to equipment and methodsof operating equipment for semiconductor fabrication. More particularly,the invention relates to methods of improving throughput in temperatureramping.

BACKGROUND OF THE INVENTION

[0003] Photoresist removal (stripping or ashing) is one of the mostfrequently applied processes in semiconductor industry. Due to thenumber of photolithographic mask steps in most fabrication processflows, it is important that ashers (strippers) attain high throughput.Removing photoresist in a vacuum chamber using an aggressive plasmachemistry is considered to be the industry standard.

[0004] Two types of system architectures for plasma ashers are widelyadopted. A first architecture employs a vacuum load-lock and a wafervacuum transfer chamber, all kept at vacuum during processing of onecassette of wafers or other workpieces. A cassette of wafers is placedin the load-lock and the load-lock is then evacuated with a vacuum pump.A wafer transfer robot then transfers the wafers from the load-lockthrough the transfer chamber to a process chamber where plasma isgenerated to remove (strip or ash) the photoresist. In the normaloperating mode, the load-lock, the wafer transfer chamber and theprocess chamber are all constantly under vacuum. After an entirecassette of wafers is processed and transferred back to the load-lock,the cassette load-lock is then vented to atmosphere, the processedwafers are removed and a new cassette of wafers is loaded.

[0005] A second system architecture transfers wafers from the cassettein an atmosphere-to-vacuum-to-atmosphere (AVA) sequence for eachindividual wafer. The robot and the wafer cassette are always atatmospheric pressure. The robot transfer wafers from the wafer cassetteto the process chamber. A vacuum pump then evacuates the process chamberto a certain vacuum level suitable for plasma formation. The plasmasource then generates plasma to remove photoresist. After the process iscomplete, the process chamber is vented to atmosphere and the processedwafer is transferred back into the cassette. This architecture entailspumping down the process chamber and venting back to atmosphericpressure for each individual wafer.

[0006] The first architecture employs two additional vacuum chambers(the load lock and transfer chambers), generally requiring an additionalvacuum pump to pump down two chambers, and therefore is much moreexpensive than the second architecture. Machines using the firstarchitecture are usually larger and occupy more clean room floor space,which is considered to be premium commodity in a semiconductorfabrication factory. Advantageously, however, the vacuum load-locksystems of the first architecture have relatively low non-productiveoverhead, exhibiting high throughput.

[0007] On the other hand, the second technique(atmospheric-to-vacuum-to-atmospheric, or AVA) involves a lower initialcapital expenditure and occupies less space on the clean room floor. Inorder to compensate for relatively higher non-productive overhead thanthe vacuum load-lock system, a variety of improvements have been made tothe AVA system. A conventional wafer processing sequence of the AVAmachine architecture is as follows:

[0008] 1. The robot transfers a new wafer or other workpiece from thecassette to the process chamber and places the wafer on support pins.

[0009] 2. The wafer is lowered onto a high temperature chuck (platen).

[0010] 3. The chuck heats the wafer up to the desired processtemperature (e.g. 250° C.).

[0011] 4. The chamber is then pumped down to a desired process ortreatment pressure (e.g. 1 Torr).

[0012] 5. The process gases start to flow and plasma is ignited by aplasma source.

[0013] 6. After the photoresist has been removed, the chamber is thenvented back to atmospheric pressure (760 Torr).

[0014] 7. The robot then exchanges the processed wafer for a fresh onefrom the cassette while transferring the processed wafer back to thecassette.

[0015] All the above steps are highly optimized to reduce the timerequired to complete each step. Heat transfer between the chuck and thewafer occurs most efficiently at atmospheric pressure; therefore, wafersare usually heated up before pumping down the chamber. In one commercialsystem, it takes about four seconds to heat a wafer from 20° C. to 250°C., at atmospheric pressure. In contrast, at a low pressure (1 Torr), ittakes about 60 seconds.

[0016] Rapid wafer heating at atmospheric pressure sometimes causeswafer warping, however, which can have a variety of negative effects.Wafer warping may damage the circuits that are already fabricated on thewafer. If a wafer is warped on a chuck, the wafer temperature is nolonger evenly distributed. A non-uniform wafer temperature distributionresults in a highly non-uniform process, since temperature is a verysensitive process variable. The wafer is usually transferred into thechamber at room temperature (20° C.). Once inside the chamber, the waferstarts to warm up somewhat while suspended above the chuck (e.g., onlift pins). To reduce the degree of thermal shock on the wafer, andconsequently prevent wafer warping, the wafer can be left suspended overthe chuck for a few seconds to pre-heat the wafer before lowering thewafer onto the high temperature chuck. The wafer descent rate can alsobe slowed so that the wafer is warming up on its way to the chuck. Bothof these options, however, also reduce throughput.

[0017] Other efforts to improve throughput focus on minimizing the timeto pump the chamber down to the treatment pressure (typically 1 Torr).Conventionally, a large vacuum pump is used for a high-speed pumping. Alarge diameter vacuum line is used to increase the pump lineconductance. Moreover, a dedicated roughing line, described in moredetail in the Detailed Description of the preferred embodiments, isintroduced to bypass the high resistance throttle valve and furtherincrease the overall pump speed during pump down. A dedicated roughingline is beneficial because the throttle valve can stay at its previousthrottling position, dramatically reducing the time to reach a stabletreatment pressure. Without the dedicated roughing line, the throttlevalve has to be wide open during pump-down to reduce the pump-down timeand then rotate to its throttling position when the process gases startto flow. This can take as much as an additional five seconds tostabilize the pressure after the chamber is pumped down.

[0018] Despite incremental improvements to the speed of each sequentialstep in the ashing process, a need exists for further improvements tothroughput for plasma asher systems.

SUMMARY OF THE INVENTION

[0019] A method is disclosed for speeding workpiece thoughput in lowpressure, high temperature semiconductor processing reactor. The methodincludes loading a workpiece into a chamber at atmospheric pressure,bringing the chamber down to an intermediate pressure, and heating thewafer while under the intermediate pressure. The chamber is then pumpeddown to the operating pressure at which substrate treatment isconducted.

[0020] The preferred embodiments involve single wafer plasma ashers,where a wafer is loaded onto lift pins at a position above a waferchuck. After placement and sealing the chamber the pressure is rapidlypumped down from a load/unload pressure (preferably atmospheric) toabout 40 Torr by rapidly opening and closing an isolation valve, and thewafer is simultaneously lowered to the heated chuck. At 40 Torr, theheat transfer from the chuck to the wafer is relatively fast, but stillslow enough to avoid thermal shock. In the interim, the pump line isfurther pumped down to operating pressure (about 1 Torr) behind theisolation valve. The chamber pressure is then again reduced by openingthe isolation valve, and the wafer is processed.

[0021] It will be understood, of course, that the preferred embodimentsare merely exemplary and that other intermediate pressures can beselected, in view of the disclosure herein, depending upon the systemand treatment process involved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a temperature vs. time graph, illustrating wafertemperature ramping at various pressures.

[0023]FIGS. 2A and 2B are schematic diagrams of a process chamber inaccordance with the preferred embodiments, including a wafer chuck withlift pins.

[0024]FIG. 3 is a schematic diagram of a vacuum pumping system inaccordance with one embodiment of the invention.

[0025]FIG. 4 is a schematic diagram of a vacuum pumping system inaccordance with another embodiment of the invention.

[0026]FIGS. 5A and 5B are graphs showing pressure equalization after anisolation valve is opened.

[0027]FIG. 6 is a flow chart illustrating a process sequence inaccordance with preferred embodiments of the present invention.

[0028]FIG. 7 is a flow chart illustrating a process sequence of anotherembodiment of the invention, particularly for removing implantedphotoresist.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] While illustrated in the context of a single wafer, remote plasmasystem, the skilled artisan will readily find application for theprinciples disclosed herein to a variety of systems in which rapidheating of a workpiece and rapid pump-down of the a chamber are desired.The invention has particular, though not exclusive, utility for plasmaashers.

[0030] Referring initially to FIG. 1, wafer-heating characteristics werestudied at various pressure levels and it was found that the rate ofheat transfer could be controlled with pressure. FIG. 1 shows threewafer heating curves at atmospheric pressure, at an intermediatepressure and at an operating pressure. In the illustrated examples,these pressures are 760 Torr, 40 Torr and 1 Torr, respectively. Allthree curves asymptotically approach 250° C. The 1 Torr curve shows avery slow heat transfer rate. The 40 Torr curve has a slightly slowerheat transfer rate than the 760 Torr, but lags only about 2 to 3 secondsbehind the 760 Torr curve. With the chuck maintained at 250° C., thewafer temperature is approximately 245° C. at steady state when thestripping (ashing) process starts.

[0031]FIGS. 2A and 2B show a process chamber 10 in accordance with thepreferred embodiments. In the illustrated embodiments, a wafer supportstructure or chuck 12 is heated (e.g., by hot fluid circulatedtherethrough or by resistance heating) to a process temperature andmaintained at that temperature during the sequential processes describedbelow. Preferably, the temperature is greater than about 150° C., morepreferably between about 200° C. and 300° C., and most preferablybetween about 225° C. and 250° C. in the illustrated plasma ashercontext.

[0032]FIG. 2A shows a wafer 12 or other workpiece in an upper position,spaced but proximate the heated wafer chuck 14. In the illustratedembodiment, the wafer 14 is held in the upper position by lift pins 16that extend through the wafer chuck 14, although the skilled artisanwill readily appreciate that the wafer can by held by a pick-up deviceor other mechanism in the upper position. The wafer 14 is generallyloaded into the chamber 10 through a gate valve 18 and supported at theupper position prior to processing, and is also unloaded from the upperposition.

[0033] The preferred system of which the chamber 10 forms a part of anatmospheric-to-vacuum-to-atmospheric (AVA) system, with attendant lowcapital costs. However, the principles and advantages disclosed hereinwill also have utility in other contexts. For example, many clustertools will have a lowered pressure for the load lock and transferchamber, and an even lower operating pressure. For such systems, theload/unload pressure may not be atmospheric, but will be higher than theoperating pressure for the treatment performed within the processchamber 10. The chamber 10 is also in fluid communication with a remoteplasma unit (not shown), though the skilled artisan will appreciate thatin situ plasma processors are also envisioned.

[0034]FIG. 2B shows the wafer 14 after being lowered to the wafer chuck12. In the illustrated embodiment, the wafer 14 is lowered bywithdrawing the lift pins 16, though in other arrangements a pick-updevice or other extraneous mechanism can lower the wafer to the chuck.In the illustrated embodiment, the wafer 14 is maintained on the chuck12 by gravitation, unaided by vacuum, electrostatic or clamping forces.Accordingly, wafer curl is of greater concern.

[0035]FIG. 3 shows a high conductance vacuum system 20 in accordancewith one embodiment of the present invention. The system 20 includes ahigh speed vacuum pump 22. An exemplary pump is the commerciallyavailable Edwards 80/1200 pump, which has a maximum pumping speed of 554cubic feet per minute (CFM). The chamber 10 of the illustratedembodiments had a volume of about 8 liters. Vacuum or pump lines 24between the pump and the chamber 10 have a length of about 20 meters anda diameter of about 100 mm. The volume of the pump lines 24 issignificantly larger than that of the chamber 10 so that it draws thechamber pressure down to the intermediate pressure of the pump lines 24quickly. Preferably, the pump lines 24 have a volume more than fivetimes larger than the process chamber 10, more preferably more than tentimes greater, and in the illustrated example the pump lines 24 arealmost 20 times greater than the process chamber.

[0036] In accordance with the illustrated embodiment, the system 20includes a throttle valve 26 and a throttling isolation valve 28.Additionally, a parallel roughing line 30, forming part of the pumplines 24, includes a roughing isolation valve 32. The operation of thethrottle and isolation valves 26, 28, 32 is described below.

[0037]FIG. 4 shows a vacuum system 20 in accordance with anotherembodiment of the present invention. In accordance with this embodiment,the roughing line and attendant roughing isolation valve are omitted.The remaining features are similar to those of FIG. 3 and areaccordingly indicated by like reference numerals.

[0038]FIG. 5A is a high pump speed pump curve, illustrating theasymptotic nature of pressure reduction with the systems of FIGS. 3 and4, while FIG. 5B shows the first few seconds of the curve in greaterdetail. As shown in FIG. 5A, it takes about 5 seconds to reach 1 Torrfrom 760 Torr. The chamber pressure drops sharply once an isolationvalve is opened. This is due to pressure equalization between thechamber and the pump line. In the illustrated example, the pump line ismaintained at about 40 Torr during wafer loading. When the isolationvalve is opened, the chamber pressure equalizes with the larger pumpline almost instantaneously. Thereafter, in the illustrated curve, theisolation valve is kept open and most of the pumping time is used todraw the vacuum from 40 Torr to the operating treatment pressure of 1Torr

[0039] The invention utilizes the heat transfer characteristic at aselected intermediate pressure (see FIG. 1) and the near instantaneouspressure equalization of the pump lines (see FIGS. 5A and 5B) to designa new process flow sequence that dramatically increases the machinethroughput. It also eliminates the wafer warping concern that is usuallyassociated with rapid wafer heating.

[0040] Standard Resist Stripping

[0041]FIG. 6 illustrates the process flow sequence of the preferredembodiments. The process will be first described with respect to theembodiment of FIG. 3, having the roughing line 30 and roughing isolationvalve 32 thereon. Reference is also made to the elements within theprocess chamber 10, schematically shown in FIGS. 2A and 2B. As will beappreciated by the skilled artisan, the wafer transfer robot(s), gatevalve, heating, gas flow and pressure control systems are desirablyprogrammed by software or hardwire for the process flows describedherein.

[0042] Initially, the wafer is loaded 100 into the upper load/unloadposition proximate the wafer chuck (i.e., onto the extended lift pins 16in the illustrated embodiment of FIG. 2A). During this time, the vacuumpump lines are maintained at or reduced to at an intermediate pressure,between the load pressure and the treatment pressure. The intermediatepressure is selected to balance the speed of thermal transfer betweenwafer and chuck with the speed of pump down to operating or treatmentpressure. Preferably, the intermediate pressure is between about 10 Torrand 700 Torr, more preferably between about 20 Torr and 100 Torr, and isabout 40 Torr in the illustrated embodiment.

[0043] Once the new wafer is transferred 100 into the chamber, the gatevalve is closed 105, the chamber pressure is then reduced 110approximately to the intermediate pressure, and the wafer is lowered 115to the heated wafer chuck. In the illustrated embodiment, pressurereduction 110 is accomplished by opening the roughing isolation valve 32(FIG. 3), while lowering 115 is simultaneously performed by withdrawingthe pins 16 (FIG. 2B) lower the wafer to the chuck quickly. Thus, steps110 and 115 are performed concurrently as one step 117 in theillustrated embodiment. Since the chamber pressure drops to theintermediate pressure (about 40 Torr) almost instantly, the roughingisolation valve 32 is then closed again right away to maintain thechamber at a pressure of between 40 to 80 Torr for wafer heating. Thetime interval between opening and closing the isolation valve determinesthe chamber pressure level. Experiments using the preferred vacuumpumping system 20 of FIG. 3 demonstrate that a 450-msec interval resultsin a pressure of around 40 Torr. Accordingly, the roughing isolationvalve 32 is preferably kept open for less than 2.0 seconds, morepreferably for about 0.5 second, and the wafer preferably lowerslinearly during the same time period from the upper position (see FIG.2A) to the lower position (see FIG. 2B). With the lowered pressure,thermal shock is avoided despite a rapid descent onto the heated chuck.

[0044] In the illustrated sequence, the wafer is preferably left on thechuck and allowed to heat up during a heating period 120. The heatingperiod 120 is longer than a standard heating period at atmosphericpressure, due to the lower pressure and consequently less efficientthermal exchange, but shorter than a low pressure (1 Torr) thermalexchange. Thus, the heating period 120 is preferably greater than about4 seconds, more preferably between about 5 seconds and 7 seconds, and isabout 6 seconds in the illustrated embodiment. The lengthened heatingperiod 120 desirably avoids thermal shock to the wafer and damage causeby wafer curling. The skilled artisan will readily appreciate thatheating can alternatively be accomplished at the intermediate pressureby more slowly lowering the wafer onto the chuck, such that the wafer isat process temperature by the time it reaches the chuck. In any case,the skilled artisan will appreciate that some amount heat is alsotransferred at other points during the process, since the chuck is heldconstant at about the process temperature. Thus, heat is transferred tothe wafer from the moment the wafer is loaded onto the lift pins in theelevated position.

[0045] During the heating period 120, the vacuum lines are evacuated bythe vacuum pump and a large vacuum reservoir is again formed behind theisolation valves. Desirably, the pump lines are pumped to the operatingpressure, which is preferably less than about 10 Torr, more preferablybetween 0.6 Torr and 2 Torr, and is about 1.0 Torr in the illustratedembodiment.

[0046] After wafer heating 120, the chamber pressure is reduced 125 tothe operating process or treatment pressure. In the present embodiment,pressure reduction 125 is again accomplished by opening the roughingvalve 32 (FIG. 3) to allow the pressure equalization. Where the vacuumline 24 is at about 1 Torr and the process chamber 10 is at about 40Torr, opening the roughing valve 32 causes the chamber pressure to dropalmost immediately to about 1.7 Torr. At this time, process or treatmentgases start to flow, the throttling isolation valve 23 opens and theroughing valve 32 closes, the throttle valve 26 starts to stabilize thechamber pressure to about 1 Torr and the plasma turns on to commencewafer processing 130 (photoresist ashing or stripping, in theillustrated embodiment). All these five control actions happen almost atthe same time.

[0047] Processing or treatment 130 will be understood to refer to thatportion of the sequence in which the workpiece is physically orchemically altered, typically by application of process or treatmentgases. In the illustrated embodiments, as noted, treatment comprises aplasma process, preferably plasma etching in the form of photoresistashing or stripping from semiconductor wafer. The skilled artisan willreadily appreciate, in view of the present disclosure, that theprinciples and advantages taught herein will have application to anumber of low pressure treatments.

[0048] After processing or treatment 130, the chamber is vented 135 andthe processed wafer is then removed or unloaded 140 from the chamber. Anew wafer is inserted or loaded 100 into the chamber and the sequencestarts again. During the chamber venting 135 and wafer transfer 140,100, the vacuum line is evacuated again and a vacuum reservoir is readyfor the new wafer.

[0049] Comparing the new sequence to a conventionalatmospheric-to-vacuum-to-atmospheric (AVA) sequence in the same system,the pump down time is almost eliminated. The wafer heating time isdesirably lengthened. In essence, the chamber pump-down and waferheating are accomplished concurrently. An exemplary processing sequencefor a conventional AVA process is as follows:

[0050] 1. The robot takes 5 seconds to remove a processed wafer from thechamber and place a new wafer in the chamber.

[0051] 2. It takes about 2 seconds to lower the wafer to the chuck toavoid warping the wafer.

[0052] It takes about 4 seconds to heat a wafer up to 245° C.

[0053] 3. It takes about 5 seconds to pump the chamber down to operatingpressure and reach stabilization.

[0054] 4. It takes about 15 seconds to complete the photoresist stripprocess,

[0055] 5. It takes about 3 seconds to vent the chamber back toatmosphere.

[0056] A total of 34 seconds is required to complete a wafer. Thisconverts to about 106 wafers per hour throughput. With the new sequence,it takes about 29 seconds to complete a wafer, translating into about124 wafers per hour throughput, which is an increase of about 18 wafersper hour (17%) without any additional cost, apart from programming thesystem for the specified sequence.

[0057] The process of FIG. 6, using the concept of partial pressureheating and pressure equalization, can also be applied without the useof the dedicated roughing line (FIG. 4). This simplifies the vacuumsystem design and reduces the cost of the machine.

[0058] According to this embodiment, the throttle valve 26 is initiallypositioned at the wide-open position with the isolation valve 28 closed.The wafer is loaded 100, the gate valve closed 105, and the waferlowered 115 to the chuck, as described above. The chamber pressure ispreferably simultaneously reduced 110 by opening the isolation valve 28and closing it immediately afterwards, as described with respect to theroughing valve of the previous embodiment. The desired intermediate orheating pressure (40 Torr in the illustrated embodiment) is thus quicklyattained.

[0059] During the heating period 120, the throttle valve 26 ispositioned to the preset throttling position for process condition. Asin the previous embodiment, the vacuum line 24 is evacuated at the sametime and a vacuum reservoir is formed.

[0060] After wafer heating 120, the chamber pressure is further reduced125 by opening the isolation valve 28 and starting pressureequalization. Process gases start to flow and the throttle valve 26starts to stabilize the pressure. Since the pressure is equalizedthrough the throttle valve 26 with a preset angle, it takes about from0.5 to 1 seconds longer to reach a treatment pressure of 1 Torrdepending on the preset throttle valve angle.

[0061] After the process 130, the chamber starts to vent 135 up toatmospheric pressure and the throttle valve 26 is set to the wide-openposition again. A typical throttle valve 26 takes about 3 to 5 secondsto go from the fully open position to fully closed position. By the timethe new wafer is inserted into the chamber, the throttle valve 26 isfully open.

[0062] This reduced hardware setup adds about 1 second to the previoussequence in the worst case. With the same process time of 15 seconds,the machine throughput is 120 wafers per hour.

[0063] Implanted Resist Stripping

[0064] As will be appreciated by the skilled artisan, some mask steps insemiconductor processing are utilized for implanting dopants intosemiconductor material. These implanted masks are particularly difficultto remove. In particular, as the wafer is heated, solvent in thephotoresist is volatilized but cannot escape due to the hardened crustleft on the resist surface by the implantation. The gases build up asthe wafer is heated and can explode, leaving a residue on the chamberwalls that is very difficult to remove.

[0065] Accordingly, it is advantageous to remove the crust of the resistat a lower temperature (e.g., 100° C. to 120° C.), then raise thetemperature to improve throughput for the remainder of the strippingprocess. Unfortunately, raising and lowering the chuck temperature,while feasible in a radiantly heated system, reduces throughputtremendously with convective/conductive or resistively heated chucks.

[0066] Referring to FIG. 7, a process for removing implanted resist isshown in accordance with a preferred embodiment of the invention. Aswith the previous embodiment, the wafer is loaded/unloaded atatmospheric pressure, and processing is conducted at low pressure(preferably less than about 10 Torr, more preferably less than about 5Torr; in the illustrated embodiment, preferably between about 0.6 Torrand 2 Torr, and more preferably at about 1.0 Torr). Also similar to theprevious embodiment, some heating is conducted at an intermediatepressure, between about 10 Torr and 700 Torr, more preferably betweenabout 20 Torr and 100 Torr, and about 40 Torr in the illustratedembodiment. Furthermore, the chuck is maintained at about the processtemperature (which is preferably greater than about 150° C., morepreferably between about 200° C. and 300° C., and most preferablybetween about 225° C. and 250° C.) throughout the process described.

[0067] Unlike the previous embodiment, heating at the intermediatepressure is conducted after partial processing.

[0068] The process can be conducted with respect to either theembodiment of FIGS. 3 or the embodiment of FIG. 4, that is, with orwithout the roughing line and roughing isolation valve thereon. Theskilled artisan will appreciate that the wafer transfer robot(s), gatevalve, heating, gas flow and pressure control systems are desirablyprogrammed by software or hardwire for the process flows describedherein.

[0069] Initially, the wafer is loaded 200 into the upper load/unloadposition proximate the wafer chuck (e.g., onto the extended lift pins 16in the illustrated embodiment of FIG. 2A). During this time, the vacuumpump lines are reduced as much as possible to an intermediate pressure,between the load pressure and the treatment pressure. Preferably, theintermediate pressure is between about 10 Torr and 700 Torr, morepreferably between about 20 Torr and 100 Torr, and is about 40 Torr inthe illustrated embodiment.

[0070] Once the new wafer is transferred 200 into the chamber, the gatevalve is closed 205, the chamber pressure is then reduced 210approximately to the treatment pressure, and the wafer is lowered 215 tothe heated wafer chuck. In the illustrated embodiment, pressurereduction 210 is accomplished by opening an isolation valve (FIGS. 3 or4), while lowering 215 is simultaneously performed by withdrawing thepins lower the wafer to the chuck quickly. Thus, steps 210 and 215 areperformed concurrently as one step 217 in the illustrated embodiment.The chamber pressure drops to the pressure of the pump lines almostinstantly, but the isolation valve remains open and pumping continuesuntil the treatment pressure is reached (preferably less than about 10Torr, more preferably less than about 5 Torr; in the illustrated plasmaashing context embodiment, preferably between about 0.6 Torr and 2 Torr,and more preferably at about 1.0 Torr). The wafer preferably lowerslinearly during the same time period from the upper or elevated positionto the lower position (see FIGS. 2A and 2B). With the low pressure, heattransfer is slow. Accordingly, the wafer tends to reach only betweenabout 100° C. and 170° C. through the subsequent process step.

[0071] Thereafter, a stripping process 220 is conducted for removing theimplanted crust of the resist at the low temperature. In addition to aconventional plasma asher chemistry including oxidant gases (e.g., 5 slmO₂ through the remote plasma unit, carrier gas), hydrogen and/orfluorine is added. For example, 1,000 sccm of 3-15% H₂ (in N₂ or He)and/or CF₄ (1-3% of total flow) can be added to the flow through thepreferred remote plasma generator. In about 20-30 seconds, the crust isremoved and the plasma flow is stopped.

[0072] After the crust is removed, the chamber pressure is elevated 225to the intermediate pressure between about 10 Torr and 700 Torr, morepreferably between about 20 Torr and 100 Torr, facilitating more rapidheat transfer between the heated chuck and the wafer. In the illustratedembodiment, the pressure is increased to 40 Torr, most preferably byflowing a high thermal conductivity gas and closing the isolation valve,thereby backfilling the chamber. Ideally, helium is utilized tofacilitate more effective thermal exchange. The pressure elevation canbe conducted at once or stepwise in incremental changes.

[0073] After a period sufficient to raise the wafer to the preferredprocess temperature (about 245° C. in the illustrated embodiment), thechamber pressure is again reduced 235 to a treatment pressure,preferably the same pressure as that of the implanted resist removalphase. The stripping 240 of standard (non-implanted) resist can thenproceed more rapidly at the elevated temperature. CF₄, if previouslysupplied, is stopped during the standard strip, although any H₂ can flowduring standard resist stripping without any harm.

[0074] After removing 240 the remainder of the resist, the chamber isvented 245 and the processed wafer is then removed or unloaded 250 fromthe chamber. A new wafer is inserted or loaded 200 into the chamber andthe sequence starts again. During the chamber venting 245 and wafertransfer 250, 200, the vacuum line is evacuated again and a vacuumreservoir is ready for the new wafer.

[0075] Conclusion

[0076] We have run the new sequence and compared the process resultswith the conventional sequence. Both results are nearly identical. Inthe conventional sequence, the rate of wafer descent onto the chuck iscritical. If the rate of descent is too fast, wafers may warp. The newsequence improves confidence because wafer warping may be eliminated asa concern due to the slower rate of heating at 40 Torr. Using the vacuumreservoir and pressure equalization concept, the extensive multitaskingcontrol sequence increases machine throughput significantly for muchimproved productivity.

[0077] Although the foregoing invention has been described in terms ofcertain preferred embodiments, other embodiments will become apparent tothose of ordinary skill in the art in view of the disclosure herein.Accordingly, the present invention is not intended to be limited by therecitation of preferred embodiments, but is intended to be definedsolely by reference to the appended claims.

We claim:
 1. A method of removing implanted photoresist from aworkpiece, the method comprising, in sequence: loading the workpieceinto a process chamber at a load/unload pressure; reducing pressure inthe process chamber to a preliminary treatment pressure and removing animplanted crust from the photoresist; raising pressure in the processchamber to an intermediate pressure between the load/unload pressure anda secondary treatment pressure; reducing pressure in the process chamberto the secondary treatment pressure; and removing non-implantedphotoresist from the workpiece at the secondary treatment pressure. 2.The method of claim 1, wherein removing the implanted crust comprisesproviding an oxidant gas and a fluorine-containing gas.
 3. The method ofclaim 2, wherein removing non-implanted photoresist comprises providingan oxidant gas without a fluorine-containing gas.
 4. The method of claim3, wherein each of the oxidant gas and the fluorine-containing gas areprovided through a remote plasma generator.
 5. The method of claim 1,wherein each of the preliminary treatment pressure and the secondtreatment pressure is less than about 5 Torr and the intermediatepressure is between about 10 Torr and 100 Torr
 6. The method of claim 1,further comprising heating workpiece by lowering the workpiece to aheated support structure while reducing the pressure in the processchamber to the secondary treatment pressure.
 7. The method of claim 6,wherein raising pressure in the process chamber further heats theworkpiece until workpiece temperature stabilizes.
 8. The method of claim7, wherein the heated support structure is maintained at a temperaturebetween about 200° C. and 300° C.
 9. The method of claim 8, whereinworkpiece reaches a temperature between about 100° C. and 170° C. afterlowering the heated support structure and prior to raising pressure inthe process chamber.
 10. The method of claim 9, wherein raising pressurein the process chamber increases workpiece temperature to between about200° C. and 250° C.